Various exhaust after-treatment devices, such as particulate filters and other devices, have been developed to effectively limit exhaust emissions from internal combustion engines.
One of the exhaust after-treatment devices frequently used in a modern lean burn internal combustion engine, such as a compression-ignition type, is a NOX adsorber or lean NOX trap (LNT). The LNT generally functions as a filter for oxides of nitrogen or NOX a gaseous emission formed as a byproduct of the reaction of nitrogen and oxygen gases in the air after combustion. The LNT removes NOX molecules from the exhaust stream by trapping and storing them internally during operation of the engine—in effect acting as a molecular sponge.
Once the LNT becomes full, no more NOX can be adsorbed by the device. In order for the LNT to adsorb more NOX, the adsorber must be purged or regenerated to restore its storage capacity. Such regeneration of the LNT is typically accomplished via injection of fuel directly into the exhaust gas that passes through the device.